This invention relates generally to linear amplifiers and more specifically to improving the performance of integrated circuit differential amplifiers.
Differential amplifiers are well known in the electronic art and are commercially available in integrated circuit (IC) packages. A typical package might have eight output pins: two for the input, one for the output, two for power connections, two for nulling the offset voltage, and one pin not connected to the circuit but included to provide an even number of pins.
The difference between signals applied to the input pins appears amplified at the output pin. Ideally, the signal at the output pin would change instantaneously in response to changes of the signals at the input pins. Components throughout the amplifier, however, have capacitance associated with them. For voltage across the components to change, current must flow to either charge or discharge those capacitances. The speed at which the output can change in response to changes at the inputs is therefore limited by the capacitance and the amount of current which flows.
The maximum rate at which the output can change in response to changes in the input is called the "slew rate". A high slew rate is desirable in many applications. In particular, where the differential amplifier must respond to rapid changes in the input signals, a high slew rate is very important. For this reason, differential amplifiers are often characterized by their slew rates.
Another characteristic of a differential amplifier is its offset voltage. Ideally, when no input signals are applied, both input pins are at the same voltage (i.e. the difference between the input pins is zero). Variations in fabricating the components of the differential amplifier, however, prevent perfect alignment of the input voltages. If the offset voltage is not corrected, incorrect output signals can result.
The nulling pins can be used to bring the input voltages into alignment. A variable resistor can be connected between those nulling pins when the amplifier is used in a circuit. The center tap of the variable resistor is connected to a power supply. Appropriately adjusting the resistor reduces the offset voltage, i.e. the offset voltage is "nulled".
One drawback to nulling the offset voltage using an external resistor is that it is inconvenient for users of differential amplifiers. For instance, electronics manufacturers mass producing products using differential amplifiers can often not afford to null each amplifier. It would be desirable, therefore, to null each amplifier when it is made.